As the United Nations Ocean Conference opened this week in Nice, France, a major scientific report revealed that one of Earth’s planetary boundaries—ocean acidification—has already been crossed. The findings mark a stark shift from previous assessments and confirm that global marine ecosystems are under far greater threat than previously understood.
The study, published in Global Change Biology and led by researchers from the United Kingdom’s Plymouth Marine Laboratory (PML), the U.S. National Oceanic and Atmospheric Administration (NOAA), and Oregon State University, found that ocean acidification (OA) crossed its critical planetary threshold by the year 2020. In some regions, that boundary was surpassed as early as 2000. The team reached their conclusion using a combination of chemical measurements, biological studies, and advanced modeling techniques.
“OA is the term given to the long-term shift of marine carbonate chemistry resulting primarily from the uptake of carbon dioxide (CO2) by the oceans, leading to an increase in ocean acidity and a decrease in carbonate ion (CO32−) concentration,” the study’s authors wrote. “OA can severely affect marine organisms through its direct impact on physiology, growth, survival and reproduction.”
Ocean acidification results from the absorption of excess atmospheric CO2 by seawater. This chemical shift decreases carbonate ion concentrations—essential for organisms that build shells or skeletons out of calcium carbonate. The process fundamentally threatens coral reefs, shellfish, and plankton species that underpin marine food webs.
The most alarming revelation in the study is the extent to which OA has infiltrated both surface and subsurface waters. By 2020, over 40% of surface ocean waters and as much as 60% of waters down to 200 meters deep had already crossed into the so-called “danger zone” of acidification.
“Looking across different areas of the world, the polar regions show the biggest changes in ocean acidification at the surface. Meanwhile, in deeper waters, the largest changes are happening in areas just outside the poles and in the upwelling regions along the west coast of North America and near the equator,” said Helen Findlay, lead author of the report, professor at Plymouth Marine Laboratory, and chair of the North-East Atlantic Ocean Acidification Hub.
“Most ocean life doesn’t just live at the surface – the waters below are home to many more different types of plants and animals. Since these deeper waters are changing so much, the impacts of ocean acidification could be far worse than we thought. This has huge implications for important underwater ecosystems like tropical and even deep-sea coral reefs that provide essential habitats and nursing refuge for many species, in addition to the impacts being felt on bottom-dwelling creatures like crabs, sea stars, and other shellfish such as mussels and oysters,” Findlay explained.
The implications for biodiversity are already measurable. The study notes that 43 percent of suitable habitat for tropical and subtropical coral reefs has been lost. In polar regions, pteropods—tiny planktonic snails also known as “sea butterflies” and vital to food webs—have lost up to 61 percent of their habitat. Coastal shellfish, including economically vital bivalves like mussels and oysters, have seen a 13 percent decline in suitable habitat globally.
The planetary boundary framework defines nine essential Earth system processes that regulate the stability and resilience of the planet. Crossing these boundaries increases the risk of irreversible environmental change. Ocean acidification now joins climate change, biosphere integrity, and several others as crossed thresholds.
“This analysis was further extended to the subsurface ocean, revealing that up to 60 percent of the global subsurface ocean (down to 200 m) had crossed that boundary, compared to over 40 percent of the global surface ocean,” the researchers wrote. “These changes result in significant declines in suitable habitats for important calcifying species, including 43 percent reduction in habitat for tropical and subtropical coral reefs, up to 61% for polar pteropods, and 13 percent for coastal bivalves.”
The economic stakes are equally daunting. Many coastal communities depend on healthy ocean ecosystems for fishing, aquaculture, and tourism. As acidification undermines shell-forming species and coral reef structures, livelihoods are increasingly at risk.
“From the coral reefs that support tourism to the shellfish industries that sustain coastal communities, we’re gambling with both biodiversity and billions in economic value every day that action is delayed,” said Steve Widdicombe, a professor at PML and co-chair of the Global Ocean Acidification Observing Network.
“Ocean acidification isn’t just an environmental crisis—it’s a ticking time bomb for marine ecosystems and coastal economies,” Widdicombe added. “As our seas increase in acidity, we’re witnessing the loss of critical habitats that countless marine species depend on and this, in turn, has major societal and economic implications.”
The findings also prompted a call to revise the previous threshold for safe levels of ocean acidification. The study suggests that a 10 percent deviation from pre-industrial carbonate chemistry levels is no longer an appropriate benchmark, as the surface ocean globally had already surpassed this stricter limit by around 2000.
In response, the researchers urge targeted conservation efforts for the species and regions most vulnerable to acidification, alongside protection for areas that remain relatively uncompromised. Preserving these ecosystems is essential for maintaining ocean resilience in the face of climate change and other human-driven pressures.
“This report makes it clear: we are running out of time and what we do—or fail to do—now is already determining our future,” said Jessie Turner, director of the International Alliance to Combat Ocean Acidification, who was not part of the study.
The report’s release comes at a pivotal moment. The United Nations Ocean Conference, co-hosted by Costa Rica and France, has drawn representatives from across the globe to discuss marine conservation amid accelerating crises. The summit’s theme—“accelerating action and mobilizing all actors to conserve and sustainably use the ocean”—echoes the urgency of the findings.
But despite the mounting scientific evidence, some major powers have pulled back from environmental leadership. Greenpeace USA oceans campaign director John Hocevar warned, “This conference couldn’t come at a more critical time. The ocean is reeling from the combined impacts of industrial fishing, plastic pollution, and climate change. And just when bold leadership is most needed, the U.S. has walked away from the global stage, opening the floodgates to destruction through a barrage of Trump administration executive orders that threaten both domestic and international waters.”
“We can’t afford any more delay,” Hocevar added. “The decisions made in Nice will set the tone for key global efforts to stem the ocean crisis in the coming months, including the plastics treaty, the global ocean treaty, and deep-sea mining talks at the International Seabed Authority. Whether this conference marks a turning point or takes our oceans further down the road to ruin will depend on the strength and ambition of the commitments made by the international community to stand up for science, uphold international law, and advance environmental justice.”
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